1. Field of the Invention
The present invention relates to a method for producing a semiconductor device, more particularly to a method for producing a semiconductor device whereby a semiconductor chip having bump electrodes formed thereupon is connected to a substrate via flip-chip connection using a thermosetting resin.
2. Description of the Prior Art
One of the known methods for producing a semiconductor device containing an LSI (large-scale integrated circuit) such as a memory or microprocessor includes flip-chip connection. The flip-chip connection consists of bonding a semiconductor chip diced from a semiconductor wafer fabricated through many varied processes, to a substrate which is so positioned as to direct its surface to carry electrodes downwards. According to this flip-chip connection it is easy to connect a chip with many electrodes to a substrate. Because this method does not require the use of bonding wires, accompaniment of unnecessary capacitance and inductance can be avoided, which is advantageous in improving the high-frequency characteristics of resulting semiconductor product.
The electrode on the surface of a chip which is subject to such flip-chip connection includes a spherical bump electrode made of gold, copper, solder or the like overlaid on a pad electrode of aluminum or the like which is prepared to facilitate the connection.
A publicly known conventional variation of such flip-chip connection consists of connecting a chip onto a substrate using a thermosetting resin as a bonding agent. This method consists of putting a bump electrode into contact with a wire via a force which develops when the thermosetting resin cools, to thereby establish electric connection between the two elements here concerned. The process required for this method is simple as compared with other variations that establish electric connection by melting a bump electrode, and thus requires a lower cost.
When flip-chip connection based on this method is put into practice, a heating means usually employed for hardening a thermosetting resin consists of passing pulse current through a semiconductor chip sucking tool, to thereby heat the chip. This is so-called pulse heating.
For example, Japanese Patent Laid-Open No. 63-151033 published on Jun. 23, 1998 disclosed a method for producing a semiconductor device which makes the use of flip-chip connection dependent on pulse-heating mentioned above. FIGS. 9 to 11 present the processes in order required by the method. Firstly, as shown in FIG. 9, a thermosetting resin 33 is applied onto a substrate 31 with wires 32 on areas to contact with a chip. Then, the chip 34 is sucked on its reverse side with a chip sucking tool 35 as shown in FIG. 10, and is transferred above the substrate 31, and a pressure is given on the chip 34 such that bump electrodes 36 are placed properly with respect to the wires 32. Through this process, excess thermosetting resin 33 is expelled outside in the presence of pressure, and the bump electrodes 36 and wires 32 come into contact with each other to establish electric connection.
Next, in this state, pulse current is flowed through the chip sucking tool 35 to heat the chip 34, and the thermosetting resin 33 hardens in the presence of heat. Finally, as shown in FIG. 11, the chip 34 is connected to the substrate 31 via flip-chip connection.
According to this type of pulse heating, it is possible to rapidly harden a thermosetting resin by heating it and thus to improve productivity.
However, with the method for producing a semiconductor device as described in said patent publication, hardening of a thermosetting resin by pulse heating takes place rapidly, which may readily cause the development of voids in the substance of thermosetting resin, which in turn may impair the reliability of resulting flip-chip connection.
To put it more specifically, conventionally with this method, to harden a thermosetting resin rapidly, it is usual to apply a large amount of energy capable of completing the hardening reaction rapidly. The resin, being subject to rapid hardening reaction, tends to develop foams. Further, if the energy were too large, the thermosetting resin would boil and develop bubbles itself. The bubbles developed by the resin may be entrapped in a space between the chip and substrate or a connector portion, and form voids there. A thermosetting resin containing voids may remain partially unhardened, where a sufficient contraction force will not develop to ensure stable electric connection.
The development of voids is also accounted for by the shape of the involved substrate. Namely, because a substrate has a wiring and on occasion a solder resist as appropriate formed thereupon, its surface can not help being rough. Because of this, when pulse heating is applied, while a pressure being imposed after a thermosetting resin has been applied, particularly indented portions of the rough surface may contain spaces not completely filled with the thermosetting resin. Air in those spaces expands with the rise of temperature, to develop voids.